Metal soaps have been industrially produced by a direct method in which a fatty acid is directly reacted with a metal oxide or a metal hydroxide, or by a double decomposition method in which a fatty acid and a basic compound are mixed and reacted with each other in the form of an aqueous solution to obtain a fatty acid basic compound salt, and then the thus obtained fatty acid basic compound salt is further reacted with a metal salt.
In the direct method, the fatty acid and the metal compound are maintained at a temperature not less than a melting point of a metal soap produced therefrom, under a solvent-free condition, so that the reaction therebetween proceeds while evaporating off water produced during the reaction out of the reaction system to thereby produce the metal soap.
The direct method have problems such as prolonged reaction time, discoloration of the metal soap owing to thermal degradation and high contents of free fatty acid and unreacted metal compound in the resultant metal soap. To solve these problems, there has been proposed the process for producing a metal soap in which a fatty acid is melted in a heating-type kneading reactor, and a metal oxide or a metal hydroxide containing water of crystallization or water adsorbed is gradually added to the molten fatty acid to allow these components to react with each other under a solvent-free condition at a temperature near a melting point of the aimed metal soap while removing water produced from the reactor (see, for example, Patent Document 1).
However, in the process proposed by the above Patent Document 1, as described in Example 2 thereof, since the amount of calcium hydroxide added is as large as 0.55 mol per mole of the fatty acid (1.1 equivalents based on the fatty acid), there still remains such a fear that the obtained metal soap contains a large amount of unreacted metal compound. Therefore, the metal soap obtained by the above process tends to be unsuitable as a food additive.
On the other hand, the double decomposition method has advantages such as a less content of free fatty acid in the obtained metal soap, facilitated dispersion of the metal soap due to fine particle size thereof, a less inclusion of different kinds of metals in the metal soap. However, these advantages usually depend upon such a production process in which a large amount of water must be used to complete the reaction, and an excess amount of the raw material which can be readily removed by purification must be charged into the reaction system in order to enhance a purity of the obtained metal soap.
More specifically, in a neutralization step for obtaining the fatty acid basic compound salt, an excess amount of the basic compound is reacted with an equivalent amount of the fatty acid.
In addition, there has also been proposed the process for producing a metal soap having a large particle size by a double decomposition method in which the aimed fatty acid metal soap is previously dispersed in an aqueous solution of a fatty acid alkali metal salt or a fatty acid ammonium salt and subjected to crystal growth to increase a particle size thereof (see, for example, Patent Document 2).
In the double decomposition method proposed by Patent Document 2 which aims at increasing a particle size of the metal soap, sodium hydroxide is reacted with the fatty acid such that the amount of sodium hydroxide used is 1.08 mol on the average per mole of the fatty acid in terms of the data described in Examples 1 to 3 thereof to obtain a fatty acid sodium salt, and then the sodium salt is further reacted with calcium chloride to obtain the aimed metal soap. Thus, in the conventional double decomposition processes, an excess amount of the basic compound has been reacted with an equivalent amount of the fatty acid.
In U.S. and European countries, the metal soaps have been used as a food additive and applied to extensive foods for the purposes of improving a moldability of foods obtained by compression-molding a powdery material into a desired shape, a fluidity of foods put on the market in the form of a powder such as a wheat flour and condiments, and an anti-aging property of bakery foods.
In Japan, the use of a fatty acid calcium salt and a fatty acid magnesium salt in foods has been past prohibited, but permitted on December, 2004 and January, 2004, respectively. As a result, the application of these salts to various foods as well as studies thereon have been now commenced.
However, the conventional metal soaps which have been contemplated to enhance a purity thereof tend to exhibit an alkalinity within a pH range of about 9 to 11 when contacted with water, since those produced by the direct method contains a large amount of residual metal oxide and metal hydroxide whereas those produced by the double decomposition method requires the use of an excess amount of a basic compound. For this reason, when the metal soaps are used as a food additive, there tends to arise such a problem that foods containing pigments, etc., suffer from discoloration when preserved for a long period of time.
Also, in the direct method, even when the reaction is conducted by using an excess amount of the fatty acid, the raw metal oxide or metal hydroxide is not completely consumed and remained unreacted owing to a low reaction rate thereof. As a result, there still remains the above problem that the metal soaps exhibit an alkalinity when contacted with water.
As described above, there have been conventionally proposed neither metal soaps having a less adverse influence on foods when added thereto as a food additive nor production process thereof.
Patent Document 1: Japanese Patent Application Laid-open No. 66551/1992.
Patent Document 2: Japanese Patent Publication No. 41658/1987.